Method and apparatus for most probable mode derivation

ABSTRACT

A method for video decoding in a decoder includes deriving for a block encoded under an intra prediction mode, a candidate list that includes a first candidate intra prediction mode value (Mode1) that corresponds to an intra prediction mode of a neighbor block that neighbors the encoded block as a first one of the plurality of intra prediction modes, and a second candidate intra prediction mode value (Mode2) and a third candidate intra prediction mode value (Mode3) that are determined in accordance with a predetermined offset from the first candidate intra prediction mode value and a modulo M operation, in which M is a power of 2 and not equal to 32. The method further includes determining an intra prediction mode value for the encoded block in accordance with the derived candidate list.

INCORPORATION BY REFERENCE

This application is a continuation of U.S. patent application Ser. No.16/147,544, filed on Sep. 28, 2018, and claims the benefit of priorityto U.S. Provisional Application No. 62/698,014, “METHODS AND APPARATUSFOR MOST PROBABLE MODE DERIVATION” filed on Jul. 13, 2018. The benefitof priority is claimed to each of the foregoing, and the entire contentsof each of the foregoing are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GByte of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce aforementioned bandwidth or storage space requirements,in some cases by two orders of magnitude or more. Both lossless andlossy compression, as well as a combination thereof can be employed.Lossless compression refers to techniques where an exact copy of theoriginal signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signal is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision contribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In Intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used in as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer bitsare required at a given quantization step size to represent the blockafter entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), benchmark set(BMS). A predictor block can be formed using neighboring samples valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

The number of possible directions has increased as video codingtechnology developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEM/VVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves can besometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

The mapping of an intra prediction directions bits in the coded videobitstream that represent the direction can be different form videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode to codewords, to complex adaptive schemes involving mostprobably modes and similar techniques. A person skilled in the art isreadily familiar with those techniques. In all cases, however, there canbe certain directions that are statistically less likely to occur invideo content than certain other directions. As the goal of videocompression is the reduction of redundancy, those less likely directionswill, in a well working video coding technology, be represented by alarger number of bits than more likely directions.

VVC is a next-generation video coding standard that is a furtherimprovement to HEVC. While the intra-prediction mode in HEVC uses 33directional modes, the intra-prediction mode in VVC uses 65 directionalmodes for improved compression. However, the conventional HEVCintra-prediction mode process is unable to process the additionaldirectional modes included in VVC.

SUMMARY

According to an embodiment of the present disclosure, there is provideda method for video decoding in a decoder includes deriving for a blockencoded under an intra prediction mode, a candidate list that includes afirst candidate intra prediction mode value (Mode₁) that corresponds toan intra prediction mode of a neighbor block that neighbors the encodedblock as a first one of the plurality of intra prediction modes, and asecond candidate intra prediction mode value (Mode₂) and a thirdcandidate intra prediction mode value (Mode₃) that are determined inaccordance with a predetermined offset from the first candidate intraprediction mode value and a modulo M operation, in which M is a power of2 and not equal to 32. The method further includes determining an intraprediction mode value for the encoded block in accordance with thederived candidate list.

According to an embodiment of the present disclosure, there is provideda video decoder that includes processing circuitry configured to derivefor a block encoded under an intra prediction mode, a candidate listthat includes a first candidate intra prediction mode value (Mode₁) thatcorresponds to an intra prediction mode of a neighbor block thatneighbors the encoded block as a first one of the plurality of intraprediction modes, and a second candidate intra prediction mode value(Mode₂) and a third candidate intra prediction mode value (Mode₃) thatare determined in accordance with a predetermined offset from the firstcandidate intra prediction mode value and a modulo M operation, in whichM is a power of 2 and not equal to 32. The processing circuitry isfurther configured to determine an intra prediction mode value for theencoded block in accordance with the derived candidate list.

According to an embodiment of the present disclosure, there is provideda non-transitory computer readable medium having instructions storedtherein, which when executed by a processor in a video decodingapparatus causes the processor to execute a method that includesderiving for a block encoded under an intra prediction mode, a candidatelist that includes a first candidate intra prediction mode value (Mode₁)that corresponds to an intra prediction mode of a neighbor block thatneighbors the encoded block as a first one of the plurality of intraprediction modes, and a second candidate intra prediction mode value(Mode₂) and a third candidate intra prediction mode value (Mode₃) thatare determined in accordance with a predetermined offset from the firstcandidate intra prediction mode value and a modulo M operation, in whichM is a power of 2 and not equal to 32. The method further includesdetermining an intra prediction mode value for the encoded block inaccordance with the derived candidate list.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a simplified block diagram of acommunication system (100) in accordance with an embodiment.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system (200) in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 5 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 6 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 7 is a schematic illustration of exemplary intra prediction modes.

FIGS. 8A and 8B illustrate the various angular modes for 35 predictionmodes.

FIGS. 9A and 9B illustrate the various angular modes for 67 predictionmodes.

FIG. 10 is a schematic illustration of a current block and itssurrounding neighbors.

FIG. 11 illustrates an embodiment of a process performed by an encoderor decoder.

FIG. 12 illustrates an embodiment of a process performed by an encoderor decoder.

FIG. 13 illustrates an embodiment of a process performed by an encoderor decoder.

FIG. 14 is a schematic illustration of a current block and neighboringblocks.

FIG. 15 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates a simplified block diagram of a communication system(100) according to an embodiment of the present disclosure. Thecommunication system (100) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (150). Forexample, the communication system (100) includes a first pair ofterminal devices (110) and (120) interconnected via the network (150).In the FIG. 1 example, the first pair of terminal devices (110) and(120) performs unidirectional transmission of data. For example, theterminal device (110) may code video data (e.g., a stream of videopictures that are captured by the terminal device (110)) fortransmission to the other terminal device (120) via the network (150).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (120) may receive the codedvideo data from the network (150), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (100) includes a secondpair of terminal devices (130) and (140) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (130) and (140)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (130) and (140) via the network (150). Eachterminal device of the terminal devices (130) and (140) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (130) and (140), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 1 example, the terminal devices (110), (120), (130) and(140) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (150) represents any number ofnetworks that convey coded video data among the terminal devices (110),(120), (130) and (140), including for example wireline (wired) and/orwireless communication networks. The communication network (150) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(150) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 2 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (213), that caninclude a video source (201), for example a digital camera, creating forexample a stream of video pictures (202) that are uncompressed. In anexample, the stream of video pictures (202) includes samples that aretaken by the digital camera. The stream of video pictures (202),depicted as a bold line to emphasize a high data volume when compared toencoded video data (204) (or coded video bitstreams), can be processedby an electronic device (220) that includes a video encoder (203)coupled to the video source (201). The video encoder (203) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (204) (or encoded video bitstream (204)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (202), can be stored on a streamingserver (205) for future use. One or more streaming client subsystems,such as client subsystems (206) and (208) in FIG. 2 can access thestreaming server (205) to retrieve copies (207) and (209) of the encodedvideo data (204). A client subsystem (206) can include a video decoder(210), for example, in an electronic device (230). The video decoder(210) decodes the incoming copy (207) of the encoded video data andcreates an outgoing stream of video pictures (211) that can be renderedon a display (212) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (204),(207), and (209) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Codingor VVC. The disclosed subject matter may be used in the context of VVC.The VVC standard is described in “Versatile Video Coding (Draft 2)”,JVET-K1001, July 2018, the entire contents of which are incorporatedherein by reference.

It is noted that the electronic devices (220) and (230) can includeother components (not shown). For example, the electronic device (220)can include a video decoder (not shown) and the electronic device (230)can include a video encoder (not shown) as well.

FIG. (3) shows a block diagram of a video decoder (310) according to anembodiment of the present disclosure. The video decoder (310) can beincluded in an electronic device (330). The electronic device (330) caninclude a receiver (331) (e.g., receiving circuitry). The video decoder(310) can be used in the place of the video decoder (210) in the FIG. 2example.

The receiver (331) may receive one or more coded video sequences to bedecoded by the video decoder (310); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (301), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (331) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (331) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (315) may be coupled inbetween the receiver (331 and an entropy decoder/parser (320) (“parser(320)” henceforth). In certain applications, the buffer memory (315) ispart of the video decoder (310). In others, it can be outside of thevideo decoder (310) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (310), forexample to combat network jitter, and in addition another buffer memory(315) inside the video decoder (310), for example to handle playouttiming. When the receiver (331) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (315) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (315) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (310).

The video decoder (310) may include the parser (320) to reconstructsymbols (321) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (310),and potentially information to control a rendering device such as arender device (312) (e.g., a display screen) that is not an integralpart of the electronic device (330) but can be coupled to the electronicdevice (330), as was shown in FIG. (3. The control information for therendering device(s) may be in the form of Supplementary EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (320) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (320) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (320) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (320) may perform entropy decoding/parsing operation on thevideo sequence received from the buffer memory (315), so as to createsymbols (321).

Reconstruction of the symbols (321) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (320). The flow of such subgroup control information between theparser (320) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (310)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (351). Thescaler/inverse transform unit (351) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (321) from the parser (320). The scaler/inversetransform unit (351) can output blocks comprising sample values, thatcan be input into aggregator (355).

In some cases, the output samples of the scaler/inverse transform (351)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (352). In some cases, the intra pictureprediction unit (352) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (358). The currentpicture buffer (358) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(355), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (352) has generated to the outputsample information as provided by the scaler/inverse transform unit(351).

In other cases, the output samples of the scaler/inverse transform unit(351) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (353) canaccess reference picture memory (357) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (321) pertaining to the block, these samples can beadded by the aggregator (355) to the output of the scaler/inversetransform unit (351) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (357) from where themotion compensation prediction unit (353) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (353) in the form of symbols (321) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (357) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (355) can be subject to variousloop filtering techniques in the loop filter unit (356). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (356) as symbols (321) from the parser (320), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (356) can be a sample stream that canbe output to the render device (312) as well as stored in the referencepicture memory (357) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (320)), the current picture buffer (358) can becomea part of the reference picture memory (357), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (310) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as document in thevideo compression technology or standard. Specifically, a profile canselect a certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (331) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (310) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 4 shows a block diagram of a video encoder (403) according to anembodiment of the present disclosure. The video encoder (403) isincluded in an electronic device (420). The electronic device (420)includes a transmitter (440) (e.g., transmitting circuitry). The videoencoder (403) can be used in the place of the video encoder (203) in theFIG. 2 example.

The video encoder (403) may receive video samples from a video source(401)(that is not part of the electronic device (420) in the FIG. 4example) that may capture video image(s) to be coded by the videoencoder (403). In another example, the video source (401) is a part ofthe electronic device (420).

The video source (401) may provide the source video sequence to be codedby the video encoder (403) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . )and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (401) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (401) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (403) may code andcompress the pictures of the source video sequence into a coded videosequence (443) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (450). In some embodiments, the controller(450) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (450) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (450) can be configured to have other suitablefunctions that pertain to the video encoder (403) optimized for acertain system design.

In some embodiments, the video encoder (403) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (430) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (433)embedded in the video encoder (403). The decoder (433) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (434). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (434) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (433) can be the same as of a“remote” decoder, such as the video decoder (310), which has alreadybeen described in detail above in conjunction with FIG. 3. Brieflyreferring also to FIG. 3, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (445) and the parser (320) can be lossless, the entropy decodingparts of the video decoder (310), including the buffer memory (315), andparser (320) may not be fully implemented in the local decoder (433).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (430) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (432) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (433) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (430). Operations of the coding engine (432) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 4), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (433) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (434). In this manner, the video encoder(403) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (435) may perform prediction searches for the codingengine (432). That is, for a new picture to be coded, the predictor(435) may search the reference picture memory (434) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(435) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (435), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (434).

The controller (450) may manage coding operations of the source coder(430), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (445). The entropy coder (445)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies known to a person skilled in the art as, for exampleHuffman coding, variable length coding, arithmetic coding, and so forth.

The transmitter (440) may buffer the coded video sequence(s) as createdby the entropy coder (445) to prepare for transmission via acommunication channel (460), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(440) may merge coded video data from the video coder (403) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (450) may manage operation of the video encoder (403).During coding, the controller (450) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of Intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A Predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A Bi-directionally Predictive Picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded non-predictively,via spatial prediction or via temporal prediction with reference to onepreviously coded reference pictures. Blocks of B pictures may be codednon-predictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (403) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (403) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (440) may transmit additional datawith the encoded video. The source coder (430) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, Supplementary EnhancementInformation (SEI) messages, Visual Usability Information (VUI) parameterset fragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to Intra prediction) makes uses of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first and a second reference picture thatare both prior in decoding order to the current picture in the video(but may be in the past and future, respectively, in display order) areused. A block in the current picture can be coded by a first motionvector that points to a first reference block in the first referencepicture, and a second motion vector that points to a second referenceblock in the second reference picture. The block can be predicted by acombination of the first reference block and the second reference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency. In the merge mode, a block inthe current picture can inherit motion vectors of a neighboring block(e.g., that shares a boundary with the block, is disposed in a largerpartition region with the block) in the current picture.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. The HEVC standardis described in H.265/HEVC (ITU-T Rec. H.265, “High Efficiency VideoCoding”, December 2016), the entire contents of which are incorporatedherein by reference (hereinafter “HEVC standard”). In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels and the like.

FIG. 5 shows a diagram of a video encoder (503) according to anotherembodiment of the disclosure. The video encoder (503) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (503) is used in theplace of the video encoder (203) in the FIG. 2 example.

In an HEVC example, the video encoder (503) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (503) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (503) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(503) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture.“Merge mode” is a submode of both inter and bi-prediction, respectively,and can be used to perform reconstruction on the prediction samples withor without the use of a residual. In other words, a coded block coded inmerge mode includes references in the form of one or more motionvectors, and perhaps control information not related to the disclosedsubject matter, and may or may not include coefficients of the residual.In certain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (503) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 5 example, the video encoder (503) includes the interencoder (530), an intra encoder (522), a residue calculator (523), aswitch (526), a residue encoder (524), a general controller (521) and anentropy encoder (525) coupled together as shown in FIG. 5.

The inter encoder (530) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique.

The intra encoder (522) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform and, in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques).

The general controller (521) is configured to determine general controldata and control other components of the video encoder (503) based onthe general control data. In an example, the general controller (521)determines the mode of the block, and provides a control signal to theswitch (526) based on the mode. For example, when the mode is the intra,the general controller (521) controls the switch (526) to select theintra mode result for use by the residue calculator (523), and controlsthe entropy encoder (525) to select the intra prediction information andinclude the intra prediction information in the bitstream; and when themode is the inter mode, the general controller (521) controls the switch(526) to select the inter prediction result for use by the residuecalculator (523), and controls the entropy encoder (525) to select theinter prediction information and include the inter predictioninformation in the bitstream.

The residue calculator (523) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (522) or the inter encoder (530). Theresidue encoder (524) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (524) is configured to convert the residuedata in the frequency domain, and generate the transform coefficients.The transform coefficients are then subject to quantization processingto obtain quantized transform coefficients.

The entropy encoder (525) is configured to format the bitstream toinclude the encoded block. The entropy encoder (525) is configured toinclude various information according to a suitable standard, such asHEVC standard. In an example, the entropy encoder (525) is configured toinclude the general control data, the selected prediction information(e.g., intra prediction information or inter prediction information),the residue information, and other suitable information in thebitstream. Note that, according to the disclosed subject matter, whencoding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 6 shows a diagram of a video decoder (610) according to anotherembodiment of the disclosure. The video decoder (610) is configured toreceive a coded pictures that are part of a coded video sequence, anddecode the coded picture to generate a reconstructed picture. In anexample, the video decoder (610) is used in the place of the videodecoder (210) in the FIG. 2 example.

In the FIG. 6 example, the video decoder (610) includes an entropydecoder (671), an inter decoder (680), a residue decoder (673), areconstruction module (674), and an intra decoder (672) coupled togetheras shown in FIG. 6.

The entropy decoder (671) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example,intra, inter, b-predicted, the latter two in merge submode or anothersubmode), prediction information (such as, for example, intra predictioninformation or inter prediction information) that can identify certainsample or metadata that is used for prediction by the intra decoder(672) or the inter decoder (680) respectively residual information inthe form of, for example, quantized transform coefficients, and thelike. In an example, when the prediction mode is inter or bi-predictedmode, the inter prediction information is provided to the inter decoder(680); and when the prediction type is the intra prediction type, theintra prediction information is provided to the intra decoder (672). Theresidual information can be subject to inverse quantization and isprovided to the residue decoder (673).

The inter decoder (680) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (672) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (673) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (673) mayalso require certain control information (to include the QuantizerParameter QP), and that information may be provided by the entropydecoder (671) (datapath not depicted as this may be low volume controlinformation only).

The reconstruction module (674) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (673) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (203), (403) and (503), and thevideo decoders (210), (310) and (610) can be implemented using anysuitable technique. In an embodiment, the video encoders (203), (403)and (503), and the video decoders (210), (310) and (610) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (203), (403) and (403), and the videodecoders (210), (310) and (610) can be implemented using one or moreprocessors that execute software instructions.

FIG. 7 illustrates an embodiment of exemplary angular modes used forintra prediction. In FIG. 7, depicted in the lower right is a subset ofnine predictor directions, which may be from H.265's 35 possiblepredictor directions. The point where the arrows converge (701)represents the sample being predicted. The arrows represent thedirection from which the sample is being predicted. For example, arrow(702) indicates that sample (701) is predicted from a sample or samplesto the upper right, at a 45 degree angle from the horizontal axis.Similarly, arrow (703) indicates that sample (701) is predicted from asample or samples to the lower left of sample (701), in a 22.5 degreeangle from the horizontal axis.

Still referring to FIG. 7, on the top right there is depicted a squareblock (704) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (704) includes 16 samples, each labelled with an “S”, itsposition in Y dimension (e.g., row index) and its position in Xdimension (e.g., column index). For example, sample S21 is the secondsample in Y dimensions (from the top) and the first (from the left)sample in X dimension. Similarly, sample S44 is the fourth sample inblock (704) in both Y and X dimension. As the block is 4×4 samples insize, S44 is at the bottom right. Further shown are reference samples,that follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (704). Intra picture prediction can work bycopying reference sample values from the neighboring samples asappropriated by the signaled prediction direction. For example, assumethe coded video bitstream includes signaling that, for this block,indicates a prediction direction consistent with arrow (702)—that is,samples are predicted from prediction sample or samples to the upperright, at a 45 degree angle from the horizontal. In that case, samplesS41, S32, S23, and S14 are predicted from same R05. Sample S44 is thenpredicted from R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

According to some embodiments, intra prediction uses previously decodedboundary samples spatially neighboring blocks to predict a newprediction block (PB). In some embodiments, angular intra prediction isemployed to efficiently model different directional structures that ispresent in video and image content. The set of available predictiondirections may be selected to provide a good trade-off between encodingcomplexity and coding efficiency.

According to some embodiments, intra prediction may be performed inaccordance with a predetermined number (X) of intra prediction modes.Each intra prediction mode may be identified with an integer, with mode0 indicating a planar intra prediction mode, and mode 1 indicating a DCintra prediction mode. The planar intra prediction mode (i.e. mode 0)may be a plane prediction mode for textured images, and may be used topreserve continuities along block edges. The DC intra prediction mode(i.e., mode 1) may be used to predict plane areas of smoothly-varyingcontent such as flat surfaces.

Modes 2 to X−1 may indicate the angular intra-prediction modes, eachangular mode indicating a direction. In video images, horizontal andvertical patterns may occur more frequently than patterns with otherdirectionalities. Accordingly, in some embodiments, intra angular modescloser to a horizontal mode or vertical mode may have smallerdisplacement parameters than displacement parameters for otherdirections. For example, the displacement parameter differences maybecome larger closer to diagonal directions to reduce a density ofprediction modes for less frequently occurring patterns.

FIGS. 8A and 8B illustrate an embodiment of 35 total intra-predictionmodes including 33 angular modes. Mode 0 is the INTRA_PLANAR mode, andmode 1 is the INTRA_DC mode. Furthermore, Modes 2-34 represent the intraangular modes. Mode 10 is the horizontal angular mode, and mode 26 isthe vertical mode.

FIGS. 9A and 9B illustrate an embodiment of 67 total intra-predictionmodes including 65 angular modes. Mode 0 is the INTRA_PLANAR mode, andmode 1 is the INTRA_DC mode. Furthermore, Modes 2-66 represent intraangular modes INTRA_ANGULAR2-INTRA_ANGULAR66, respectively. Mode 18 isthe horizontal mode, and mode 50 is the vertical mode.

The embodiment illustrated in FIGS. 9A and 9B enables the capturing ofarbitrary edge directions presented in natural video. Furthermore, theembodiment illustrated in FIGS. 9A and 9B provides better predictionaccuracy than the embodiment illustrated in FIGS. 8A and 8B due to theincreased number of modes. However, during a most probable mode (MPM)derivation process, when additional modes are obtained by adding −1 or+1 to the angular modes that are already included in an MPM list, amodulo operation %65 needs to be performed. The %65 operation isdisadvantageous because this operation is more complicated to implementin hardware, such as an encoder or decoder, than a %2^(N) operation. Inthis regard, the modulo operation complexity is similar to thecomplexity of the division operation, which is not desired by one ofordinary skill in the art when performing video encoding and decoding.However, the modulo operation on 2^(N) can be performed in a differentway as & (2^(N)), which is simpler than the division operation.

According to some embodiments, when deriving the neighboring modes of agiven angular mode, a modulo % M operation is used, where M is a power2, but may not be equal to 32. The neighboring modes may be derivedduring an MPM derivation process, the secondary MPM derivation, or theP^(th) MPM list derivation. Examples of M include, but are not limitedto 2, 4, 8, 16, 64, 128, 256, etc.

According to some embodiments, the total number of angular intra modesis K=1+2^(N), the modulo M value is 2^(N), and the angular mode indexranges from A₀ to A_(K-1). The −d mode may be derived as follows:

M_0=((mode+2^(N) −A ₀ −d)%2^(N))+A ₀.  Eq. 1:

The +d mode may be derived as follows:

M_1=((mode−A ₀ +d)%2^(N))+A ₀.  Eq. 2:

As an example, when M is equal to 64 (e.g., N=6), the total number ofangular intra modes K is 1+2⁶=65, A₀ is 2, and A_(K-1) is 64. The −dmode is derived as follows:

M_0=((mode+62−d)%64)+2.

Furthermore, the +d mode is derived as follows:

M_1=((mode−2+d)%64)+2.

According to some embodiments, M is a sum of multiple (X) powers of 2.Example values of M include, but are not limited to, 32+64, 32+16,16+64, etc. Example values of X include, but are not limited to 2, 3, 4,etc. When M is a sum of multiple (X) powers of 2, the total number ofangular intra modes is K=1+2^(N0)+2^(N1), where M is 2^(N0)+2^(N1), andthe angular mode index ranges from A₀ to A_(K-1).

The −d mode may be derived as follows:

M_0=((mode+2^(N0)+2^(N1) −A ₀ −d)%(2^(N0)+2^(N1)))+A ₀.  Eq. 3:

The +d mode may be derived as follows:

M_1=((mode+2^(N0)+2^(N1) −A ₀ +d)%(2^(N0)+2^(N1)))+A ₀.  Eq. 4:

According to some embodiments, to accommodate for the increased numberof directional intra modes, an intra mode coding method derives acandidate list that includes a predetermined number of Most ProbableModes (MPMs). In some embodiments, the predetermined number is 6, but asunderstood by one of ordinary skill in the art, is not limited to thisnumber of MPMs. The modes included in the candidate list may beclassified as (1) neighbor intra modes, (2) derived intra modes, and (3)default intra modes.

In some embodiments, five neighboring intra prediction modes are used toform an initial candidate list. FIG. 10 illustrates a current block 1001surrounded by neighboring blocks (i) below left (BL) 1002, (ii) left1003, (iii) above right (AR) 1004, (iv) above (A) 1005, and (v) aboveleft (AL) 1006. The order of these blocks in the initial candidate listis left, above, planar, DC, below-left, above-right, and above-left.

In some embodiments, after an initial candidate list is formed with themodes of the neighboring blocks illustrated in FIG. 10, a pruningprocess is used to remove duplicated modes so that unique modes areremaining in the candidate list. For example, if the left block (1003)and below left block (1002) are both the INTRA_DC mode, one of theseblocks is removed from the candidate list.

If the candidate list is not full after the pruning process (e.g., thereare less than 6 MPM candidates in the candidate list), derived modes areadded to the candidate list. The derived modes may be obtained by adding−1 or +1 to each intra angular node included in the candidate list. Forexample, with d=1, K=66, and N=6, Eq. 1 or 3 may be used to derive the−1 mode, and Eq. 2 or 4 may be used to derive the +1 mode. For example,when Eq. 1 is used, the −d mode is:

M_0=((mode+62−d)%64)+2.

Furthermore, when Eq. 2 is used, the +d mode is:

M_1=((mode−2+d)%64)+2.

In the above examples d may be a positive integer such as d=1, 2, 3,etc.

If the candidate list is still not full after adding the derived modesto the candidate list, default modes may be added in the followingorder: vertical mode, horizontal mode, mode 2, and diagonal mode. As aresult of this process, a unique candidate list of 6 MPM modes isgenerated.

According to some embodiments, for entropy coding of the selected modeusing the 6 MPMs, a truncated unary binarization is used. The firstthree bins may be coded with contexts that depend on the MPM moderelated to the bin currently being signaled. The MPM mode may beclassified into one of three categories: (a) modes that arepredominantly horizontal (i.e., the MPM mode number is less than orequal to the mode number for the diagonal direction), (b) modes that arepredominantly vertical (i.e., the MPM mode is greater than the modenumber for the diagonal direction), and (c) the non-angular (DC andplanar) class. Accordingly, three contexts may be used to signal the MPMindex based on this classification.

According to some embodiments, the coding for selection of the remaining61 non-MPMs may be performed as follows. The 61 non-MPMs may be firstdivided into two sets: a selected mode set and a non-selected mode set.The selected modes set may contain 16 modes and the rest (45 modes) maybe assigned to the non-selected modes set. The mode set that the currentmode belongs to may be indicated in the bitstream with a flag. If themode to be indicated is within the selected mode set, the selected modemay be signaled with a 4-bit fixed-length code, and if the mode to beindicated is from the non-selected set, the selected mode may besignaled with a truncated binary code.

FIG. 11 illustrates an embodiment of a process performed by an encoderor decoder such as intra encoder 522 or intra decoder 672, respectively.The process illustrated in FIG. 11 may be used to derive a candidatelist that is used to determine an intra prediction mode for a currentblock. The process may start a step S1100 with an empty candidate list,where for a current block, neighbor intra modes are added to a candidatemode list. For example, referring to FIG. 10, for current block 1001,the intra modes of neighboring blocks 1002-1006 are added to thecandidate list. The process proceeds to step S1102, where duplicatedintra modes are removed from the candidate list. For example, if twomodes in the candidate list both include the mode INTRA_DC, one of thesemodes is removed.

The process proceeds to step S1104 where it is determined whether thenumber of modes in the candidate mode list is less than a predeterminednumber. For example, the predetermined number may be set to 6. If thenumber of nodes in the candidate list is equal to the predeterminednumber, the candidate list is completed, and the process proceeds tostep S1112 where the intra mode for the current block is determined forthe current block using the candidate list.

If the number of modes in the candidate list is less than thepredetermined number, the process proceeds to step S1106 to determinethe derived intra modes. For example, for each intra angular modeincluded in the candidate list, the −1 mode is determined using Eq. 1 or3 with d=1, and the +1 mode is determined using Eq. 2 or 4 with d=1.

The process proceeds to step S1108 to determine whether the number ofmodes in the candidate list is less than the predetermined number. Ifthe number of modes in the candidate list is still less than thepredetermined number, the process proceeds to step S1110 where defaultnodes are added to the candidate list. For example, the default modesmay include the vertical mode, horizontal mode, mode 2, and the diagonalmode. The process proceeds from step S1110 to step S1112. Returning tostep S1108, if the number of modes in the candidate list is equal to thepredetermined number, the process proceeds to step S1112.

FIG. 12 illustrates an embodiment of a process performed by an encoderor decoder such as intra encoder 522 or intra decoder 672, respectively,for determining a luma intra prediction mode for a current block. Theprocess illustrated in FIG. 12 may be performed using the intraprediction modes illustrated in FIG. 9 where there are 67 total intraprediction modes with the intra angular modes numbered from 2-66. Theprocess illustrated in FIG. 12 may start at step S1200, whereneighboring locations are set for a current block. For example if thecurrent block has location coordinates (xPb, yPb), the neighboringlocations (xNbA, yNba) and (xNbB, yNbB) is set as follows:

(xNbA, yNba)=(xPb−1, yPb),

(xNbB, yNbB)=(xPb, yPb−1).

FIG. 14 illustrates examples of the neighboring locations (xNbA, yNbA)and (xNbB, yNbB). For example, block A represents neighboring location(xNbA, yNba) since this neighboring location is to the left of thecurrent block (xPb, yPb). Furthermore, block B represents neighboringlocation (xNbB, yNbB) since this neighboring location is above thecurrent block (xPb, yPb).

The process proceeds to step S1202 where the intra prediction modes forthe neighboring locations are determined. For example, the intraprediction modes for neighboring locations (xNbA, yNba) and (xNbB, yNbB)may be assigned to variables candIntraPredModeA and candIntraPredModeB,respectively. In this regard, candIntraPredModeA may represent the intraprediction mode for block A (FIG. 14) and candIntraPredModeB mayrepresent the intra prediction mode for block B (FIG. 14). The variablescandIntraPredModeA and candIntraPredModeB may be generally referred toas candIntraPredModeX. The following steps (i) and (ii)(a)-(d) describean example process for determining the variable candIntraPredModeX, ForX being replaced by either A or B, the variable candIntraPredModeX(e.g., candIntraPredModeA or candIntraPredModeB) is derived as follows:

-   -   (i) The process begins by assigning a value to a parameter        (xCurr, yCurr) as a current location for a current block, and        then determining whether reference samples are available for the        current block. For example, the availability derivation process        for a block in z-scan order as specified in the HEVC standard,        clause 6.4.1 is invoked with the location (xCurr, yCurr) set        equal to (xPb, yPb) and the neighbouring location (xNbY, yNbY)        set equal to (xNbX, yNbX) as inputs, and the output is assigned        to a boolean parameter availableX, which is used to indicate        whether reference samples for the current block are available.    -   (ii) The candidate intra prediction mode candIntraPredModeX is        derived as follows:        -   (a) If availableX is equal to FALSE (i.e., reference sample            is not available), candIntraPredModeX is set equal to            INTRA_DC,        -   (b) Otherwise (i.e., reference sample is available), if            CuPredMode[xNbX][yNbX] is not equal to MODE_INTRA or            pcm_flag[xNbX][yNbX] is equal to 1, candIntraPredModeX is            set equal to INTRA_DC,        -   (c) Otherwise, if X is equal to B and yPb-1 is less than            ((yPb>>CtbLog2SizeY)<<CtbLog2SizeY), candIntraPredModeB is            set equal to INTRA_DC.        -   (d) Otherwise, candIntraPredModeX is set equal to            IntraPredModeY[xNbX] [yNbX]

In the above example, the variable CuPredMode may represent the intraprediction mode of the current block, the variable pcm_flag may indicatewhether the differential pulse code modulation (DPCM) mode is enabled ornot, and the variable CtbLog2SizeY may represent the Log2 value of theblock height.

The process proceeds to step S1204 where the candidate list of mostprobable modes (MPMs) is determined. An example process for determiningthe candidate list of MPMs is illustrated in FIG. 13. In FIG. 13, thevariable A represents candIntraPredModeA, and the variable B representscandIntraPredModeB.

The process illustrated in FIG. 13 may start at step S1300 where it isdetermined whether A=B. If A=B (i.e., A and B have the same intraprediction mode), the process proceeds to step S1302 to determinewhether A<2 (i.e., A is a non-angular intra prediction mode). If A<2,the process proceeds to step S1304, where the candidate list isdetermined as follows:

candlist[0]=Intra_Planar

candlist[1]=Intra_DC

candlist [2]=IntraAngular50.

If A is not less than 2, the process proceeds from step S1302 to stepS1306, where the candidate list is determined as follows:

-   -   candlist[0]=A candlist[1]=2+((A+61) % 64) candlist[2]=2+((A−2+1)        % 64).

As illustrated above, the first entry in the candidate list (e.g.,candlist[0]) is set to inter prediction mode corresponding to block A(FIG. 14), the second entry in the candidate list (e.g., candlist[1]) isdetermined in accordance with Eq. 1, and the third entry in thecandidate list (e.g., candlist [2]) is determined in accordance with Eq.2 with d=1, N=6, and A₀=2.

Returning to step S1300, if A is not equal to B, the process proceeds tostep S1308 where the first entry in the candidate list (e.g.,candlist[0]) is set to A and the second entry in the candidate list(e.g., candlist[1]) is set to B. The process proceeds to step S1310where it is determined whether A and B are both not equal to theIntra_Planar mode. If A and B are both not equal to the Intra_Planarmode, the process proceeds to step S1312 where the third entry in thecandidate list (e.g., candlist[2]) is set equal to the Intra_Planarmode.

Returning to step S1310, if A or B is equal to the Intra_Planar mode(e.g., mode 0), the process proceeds to step S1314 where it isdetermined whether A and B are both not equal to the Intra_DC mode(e.g., mode 1). If A and B are both not equal to the Intra_DC mode, theprocess proceeds to step S1316 where the third entry in the candidatelist (e.g., candlist[2]) is set equal to the Intra_DC mode. However, ifA or B is equal to the Intra_DC mode, the process proceeds from stepS1314 to step S1318 where the third entry in the candidate list (e.g.,candlist[2]) is set to the vertical intra prediction mode (e.g., mode50).

After the candidate list of MPMs is created, the process illustrated inFIG. 13 is completed. Returning to FIG. 12, the process proceeds fromstep S1204 to step S1206, where the intra prediction mode for thecurrent block is determined. The intra prediction mode for the currentblock may be represented by the variable IntraPredModeY[xPb][yPb], whichmay be determined in accordance with steps (i) and (ii)(a)-(e) as setforth below. Particularly, in step (i) it is determined whether theintra prediction mode of the current block is include in the candidatelist (e.g., boolean parameter prev_intra_luma_pred_flag is true). Forexample, the variable prev_intra_luma_pred_flag[xPb] [yPb] may be set to1 by the encoder if the luma intra prediction mode of the current blockis one of the three candidates included in the candidate list, which maybe indicated by the variable mpm_idx. The variablesprev_intra_luma_pred_flag and mpm_idx may be signaled by the encoder tothe decoder in a video bitstream.

If the inter prediction mode is not included in the candidate list(e.g., prev_intraluma_pred_flag[xPb][yPb]=0), then in step (ii) thecandidate list is ordered from lowest to highest (e.g., steps (a)-(c)),then the inter prediction mode for the current block is set to a non-MPMmode (e.g., rem_intra_luma_pred_mode) and derived in accordance with thederived candidate list (e.g., steps (d) and (e).

The process for determining the intra prediction mode for the currentblock (e.g., IntraPredModeY[xPb][yPb]) may be described as follows:

-   -   (i) If prev_intra_luma_pred_flag[xPb][yPb] is equal to 1, the        IntraPredModeY[xPb][yPb] is set equal to        candModeList[mpm_idx[xPb][yPb]],    -   (ii) Otherwise, IntraPredModeY[xPb][yPb] is derived by applying        the following ordered steps:        -   (a) When candModeList[0] is greater than candModeList[1],            both values are swapped as follows:        -   (candModeList[0], candModeList[1])=Swap(candModeList[0],            candModeList[1]),        -   (b) When candModeList[0] is greater than candModeList[2],            both values are swapped as follows:        -   (candModeList[0], candModeList[2])=Swap(candModeList[0],            candModeList[2]),        -   (c) When candModeList[1] is greater than candModeList[2],            both values are swapped as follows:        -   (candModeList[1], candModeList[2])=Swap(candModeList[1],            candModeList[2])        -   (d) IntraPredModeY[xPb][yPb] is set equal to            rem_intra_luma_pred_mode[xPb] [yPb],        -   (e) For i equal to 0 to 2, inclusive, when            IntraPredModeY[xPb][yPb] is greater than or equal to            candModeList[i], the value of IntraPredModeY[xPb][yPb] is            incremented by one.

The techniques and processes described above, can be implemented ascomputer software using computer-readable instructions and physicallystored in one or more computer-readable media. For example, FIG. 15shows a computer system (1500) suitable for implementing certainembodiments of the disclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 15 for computer system (1500) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1500).

Computer system (1500) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1501), mouse (1502), trackpad (1503), touchscreen (1510), data-glove (not shown), joystick (1505), microphone(1506), scanner (1507), camera (1508).

Computer system (1500) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1510), data-glove (not shown), or joystick (1505), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1509), headphones(not depicted)), visual output devices (such as screens (1510) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability-some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1500) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1520) with CD/DVD or the like media (1521), thumb-drive (1522),removable hard drive or solid state drive (1523), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1500) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1549) (such as, for example USB ports of thecomputer system (1500)); others are commonly integrated into the core ofthe computer system (1500) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1500) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1540) of thecomputer system (1500).

The core (1540) can include one or more Central Processing Units (CPU)(1541), Graphics Processing Units (GPU) (1542), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1543), hardware accelerators for certain tasks (1544), and so forth.These devices, along with Read-only memory (ROM) (1545), Random-accessmemory (1546), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1547), may be connectedthrough a system bus (1548). In some computer systems, the system bus(1548) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1548),or through a peripheral bus (1549). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1541), GPUs (1542), FPGAs (1543), and accelerators (1544) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1545) or RAM (1546). Transitional data can be also be stored in RAM(1546), whereas permanent data can be stored for example, in theinternal mass storage (1547). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1541), GPU (1542), massstorage (1547), ROM (1545), RAM (1546), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1500), and specifically the core (1540) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1540) that are of non-transitorynature, such as core-internal mass storage (1547) or ROM (1545). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1540). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1540) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1546) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1544)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

Appendix A: Acronyms

-   MV: Motion Vector-   HEVC: High Efficiency Video Coding-   SEI: Supplementary Enhancement Information-   VUI: Video Usability Information-   GOPs: Groups of Pictures-   TUs: Transform Units,-   PUs: Prediction Units-   CTUs: Coding Tree Units-   CTBs: Coding Tree Blocks-   PBs: Prediction Blocks-   HRD: Hypothetical Reference Decoder-   SNR: Signal Noise Ratio-   CPUs: Central Processing Units-   GPUs: Graphics Processing Units-   CRT: Cathode Ray Tube-   LCD: Liquid-Crystal Display-   OLED: Organic Light-Emitting Diode-   CD: Compact Disc-   DVD: Digital Video Disc-   ROM: Read-Only Memory-   RAM: Random Access Memory-   ASIC: Application-Specific Integrated Circuit-   PLD: Programmable Logic Device-   LAN: Local Area Network-   GSM: Global System for Mobile communications-   LTE: Long-Term Evolution-   CANBus: Controller Area Network Bus-   USB: Universal Serial Bus-   PCI: Peripheral Component Interconnect-   FPGA: Field Programmable Gate Areas-   SSD: solid-state drive-   IC: Integrated Circuit-   CU: Coding Unit

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

The above disclosure also encompasses the embodiments listed below:

(1) A method for video decoding in a decoder includes deriving for ablock encoded under an intra prediction mode, a candidate list thatincludes a first candidate intra prediction mode value (Mode₁) thatcorresponds to an intra prediction mode of a neighbor block thatneighbors the encoded block as a first one of the plurality of intraprediction modes, and a second candidate intra prediction mode value(Mode₂) and a third candidate intra prediction mode value (Mode₃) thatare determined in accordance with a predetermined offset from the firstcandidate intra prediction mode value and a modulo M operation, in whichM is a power of 2 and not equal to 32, and determining an intraprediction mode value for the encoded block in accordance with thederived candidate list.

(2) The method according to feature (1), in which Mode₂ is determined inaccordance with: ((Mode₁+M−A₀−d) % M)+A₀, wherein N is a positiveinteger, d is a positive integer, a total number of angular intra modesis K=1+2^(N), M is equal to 2^(N), and an angular mode index ranges fromA₀ to A_(k-1).

(3) The method according to feature (2), in which Mode₃ is determined inaccordance with ((Mode₁−A₀+d) % M)+A₀.

(4) A video decoder includes processing circuitry configured to: derivefor a block encoded under an intra prediction mode, a candidate listthat includes a first candidate intra prediction mode value (Mode₁) thatcorresponds to an intra prediction mode of a neighbor block thatneighbors the encoded block as a first one of the plurality of intraprediction modes, and a second candidate intra prediction mode value(Mode₂) and a third candidate intra prediction mode value (Mode₃) thatare determined in accordance with a predetermined offset from the firstcandidate intra prediction mode value and a modulo M operation, in whichM is a power of 2 and not equal to 32, and determine an intra predictionmode value for the encoded block in accordance with the derivedcandidate list.

(5) The video decoder according to feature (4), in which Mode₂ isdetermined in accordance with: ((Mode₁+M−A₀−d) % M)+A₀, in which N is apositive integer, d is a positive integer, a total number of angularintra modes is K=1+2^(N), M is equal to 2^(N), and an angular mode indexranges from A₀ to A_(k-1).

(6) The video decoder according to feature (5), in which Mode₃ isdetermined in accordance with: ((Mode₁−A₀+d) % M)+A₀.

(7) A non-transitory computer readable medium having instructions storedtherein, which when executed by a processor in a video decodingapparatus causes the processor to execute a method that includesderiving for a block encoded under an intra prediction mode, a candidatelist that includes a first candidate intra prediction mode value (Mode₁)that corresponds to an intra prediction mode of a neighbor block thatneighbors the encoded block as a first one of the plurality of intraprediction modes, and a second candidate intra prediction mode value(Mode₂) and a third candidate intra prediction mode value (Mode₃) thatare determined in accordance with a predetermined offset from the firstcandidate intra prediction mode value and a modulo M operation, in whichM is a power of 2 and not equal to 32; and determining an intraprediction mode value for the encoded block in accordance with thederived candidate list.

(8) The non-transitory computer readable medium according to feature(7), wherein Mode₂ is determined in accordance with:

((Mode₁ +M−A ₀ −d)% M)+A ₀,

wherein N is a positive integer, d is a positive integer, a total numberof angular intra modes is K=1+2^(N), M is equal to 2^(N), and an angularmode index ranges from A₀ to A_(k-1).

(9) The non-transitory computer readable medium according to feature(8), wherein Mode₃ is determined in accordance with:

((Mode₁ −A ₀ +d)% M)+A ₀.

1. (canceled)
 2. A method for video decoding in a decoder, comprising:deriving for a block encoded under an intra prediction mode, using amost probable mode (MPM) derivation process, a candidate list thatincludes: a first candidate intra prediction mode value (Mode₁) thatcorresponds to an intra prediction mode of a neighbor block thatneighbors the encoded block as a first one of the plurality of intraprediction modes, and a second candidate intra prediction mode value(Mode₂) and a third candidate intra prediction mode value (Mode₃) thatare determined in accordance with a predetermined offset from the firstcandidate intra prediction mode value and a modulo M operation, in whichM is a sum of multiple powers of 2 and not equal to 32; and determiningan intra prediction mode value for the encoded block in accordance withthe derived candidate list.
 3. The method according to claim 2, whereinMode₂ is determined in accordance with:((Mode₁ +M−A ₀ −d)% M)+A ₀, wherein N is a positive integer, d is apositive integer, a total number of angular intra modes is K=1+2^(N), Mis equal to 2^(N0)+2^(N1), and an angular mode index ranges from A₀ toA_(k-1).
 4. The method according to claim 3, wherein Mode₃ is determinedin accordance with:((Mode₁ −A ₀ +d)% M)+A ₀.
 5. A video decoder for video decoding,comprising: circuitry configured to: derive for a block encoded under anintra prediction mode, using a most probable mode (MPM) derivationprocess, a candidate list that includes: a first candidate intraprediction mode value (Mode₁) that corresponds to an intra predictionmode of a neighbor block that neighbors the encoded block as a first oneof the plurality of intra prediction modes, and a second candidate intraprediction mode value (Mode₂) and a third candidate intra predictionmode value (Mode₃) that are determined in accordance with apredetermined offset from the first candidate intra prediction modevalue and a modulo M operation, in which M is a sum of multiple powersof 2 and not equal to 32; and determine an intra prediction mode valuefor the encoded block in accordance with the derived candidate list. 6.The video decoder according to claim 5, wherein Mode₂ is determined inaccordance with:((Mode₁ +M−A ₀ −d)% M)+A ₀, wherein N is a positive integer, d is apositive integer, a total number of angular intra modes is K=1+2^(N), Mis equal to 2^(N0)+2^(N1), and an angular mode index ranges from A₀ toA_(k-1).
 7. The video decoder according to claim 5, wherein Mode₃ isdetermined in accordance with:((Mode₁ −A ₀ +d)% M)+A ₀.
 8. A non-transitory computer readable mediumhaving instructions stored therein, which when executed by a processorin a video decoder causes the processor to execute a method comprising:deriving for a block encoded under an intra prediction mode, using amost probable mode (MPM) derivation process, a candidate list thatincludes: a first candidate intra prediction mode value (Mode₁) thatcorresponds to an intra prediction mode of a neighbor block thatneighbors the encoded block as a first one of the plurality of intraprediction modes, and a second candidate intra prediction mode value(Mode₂) and a third candidate intra prediction mode value (Mode₃) thatare determined in accordance with a predetermined offset from the firstcandidate intra prediction mode value and a modulo M operation, in whichM is a sum of multiple powers of 2 and not equal to 32; and determiningan intra prediction mode value for the encoded block in accordance withthe derived candidate list.
 9. The non-transitory computer readablemedium according to claim 8, wherein Mode₂ is determined in accordancewith:((Mode₁ +M−A ₀ −d)% M)+A ₀, wherein N is a positive integer, d is apositive integer, a total number of angular intra modes is K=1+2^(N), Mis equal to 2^(N0)+2^(N1), and an angular mode index ranges from A₀ toA_(k-1).
 10. The non-transitory computer readable medium according toclaim 9, wherein Mode₃ is determined in accordance with:((Mode₁ −A ₀ +d)% M)+A ₀.